As was said it depends first on whether you plan to sit in a chair, use a butt-jack (stool), or simply stand. I HIGHLY recommend sitting in a rolly office chair, secretary-type with the vertical back that forces you not to slump your shoulders.
What I did for my indoor casting setup is build the bench as low as possible yet still get my knees under it. I did that with 2x12 dimensional lumber and no bracing where my knees go, the top is about 27" off the floor. For a topper I used 1/2" sheet rock. The paper will singe slightly if you pour hot lead directly on it, but it's a nice surface, easily replaced, cheap, and virtually fireproof. Sheet rock won't damage a mould if you drop it. I elevated my furnace on a stack of fire bricks so I can see the stream pour, but not so high that it fatigues my shoulders excessively. I throw a folded towel down next to the furnace, put a cull bin directly in front of me, and have my hot plate with mould oven next to the furnace opposite the towel. For ventilation I built a fume hood out of wood and used 6" steel ductwork to connect a remote, in-line 500 CFM exhaust fan in the attic.
When I water-quench, I simply put a bucket full of water next to me on the floor and turn my body at a slight angle to the pot. The rolly chair lets me kick myself quickly back out of harm's way should the need arise.